The present invention relates to the lighting arts. It especially relates to light emitting diodes with inverted flip-chip orientations, and will be described with particular reference thereto. However, the invention will also find application in conjunction with non-inverted light emitting diodes, laser diodes, and the like.
Light emitting diodes are generally low voltage devices having operating voltages of a few volts. For example, a typical gallium nitride-based light emitting diode has a forward operating voltage of about 3-4 volts. Low voltage devices have certain disadvantages at the system level. Voltage dividers, power regulators, or other power conditioning circuitry may be needed to reduce high voltage electrical power to a level suitable for driving the light emitting diodes. Moreover, the low voltage calls for relatively high operating current to achieve high brightness. Since resistive power losses scale with current-squared, increasing current can reduce light conversion efficiency. Moreover, since interconnect voltage drops per unit length of wire or printed circuit trace increases proportionally with current, high operating currents can limit the spatial separation between the power source and the light emitting diode.
At the die level, low voltage operation is also problematic. The forward operating voltage is determined largely by the bandgap of the semiconductor material, which is substantially constant for a given material. Additional resistive voltage drops due to resistance of the semiconductor layers (and of the substrate if it is part of the current path) also contribute to the forward voltage. However, resistive voltage drops generally do not substantially increase light output intensity, since some of the energy is going into resistive heating. Furthermore, the resistive heating can increase the resistivity of the semiconductor materials, further exasperating the resistance problem. Resistive heating can produce other undesirable effects such as thermal device failure. In summary, the low voltage, high current nature of light emitting diodes limits the magnitude of light output that can be achieved with a single diode junction.
One approach for overcoming these low voltage issues is to series-interconnect a plurality of light emitting diodes. Series interconnection can be done at the system level by connecting a plurality of discrete light emitting diode components in series. However, such component-level series interconnection increases system complexity and produces a spatially distributed light source. Additionally, failure of one device in the series breaks the current path and can cause failure of the entire series-interconnected chain of light emitting diodes.
It is also known to series-interconnect a plurality of light emitting diode mesas on a single substrate to provide a light emitting diode die with monolithically integrated series-interconnected light emitting diode mesas. Such light emitting diode devices are described, for example, in Steigerwald et al., U.S. Pat. No. 6,307,218, and in Collins, III et al., U.S. Pat. No. 6,547,249.
However, these light emitting diode devices have certain disadvantages. The devices typically exhibit non-uniform current distribution because current is injected from the mesa side and spreads non-uniformly over the mesa area. Such current non-uniformity can reduce light output efficiency and can introduce thermal hotspots. Moreover, the entire device die may degrade in performance or catastrophically fail due to a current flow problem with any single mesa, or with any single series interconnection in the series-interconnected chain of light emitting diode mesas. A relatively long overall current path of the device also leads to increased resistive voltage drop, increased resistive current-squared power losses, and consequent reduction in light output efficiency. Still further, the electrodes of the die are relatively large to accommodate the power input needed to drive the plurality of mesas in series, and these large electrodes are disposed outside the light emitting area, which substantially reduces the active area of the light emitting diode die.
The present invention contemplates an improved apparatus and method that overcomes the above-mentioned limitations and others.